1. Field of the Invention
The present invention relates to a direct conversion circuit for directly outputting a baseband signal from a mixer circuit.
2. Description of the Related Art
A conventional direct conversion circuit is described with reference to FIG. 3. A reception signal 131 received by an antenna 101 is input to mixers 103 and 104. Also, cosine waves 133 and sine waves 134 are output from a phase shifter 102, to which a local oscillation signal 132 (Lo) is input, and are input to the mixers 103 and 104, respectively. The mixer 103 outputs a signal 135 by mixing the reception signal 131 and the cosine waves 133 and performing downconversion, while the mixer 104 outputs a signal 136 by mixing the reception signal 131 and the sine waves 134 and performing downconversion.
Next, after direct-current offsets are eliminated by AC couplings 105 and 106, the signals 135 and 136 are output as signals 137 and 138, and after unnecessary frequency components are eliminated, the signals 137 and 138 are output as signals 139 and 140. The signals 139 and 140 are amplified to predetermined levels by amplifiers 109 and 110, and their output signals 141 and 142 are shaped in waveform by analog root Nyquist filters 111 and 112, whereby a baseband I signal 143 and a baseband Q signal 144 having a shaped spectrum are obtained. The baseband I signal 143 and the baseband Q signal 144 are converted into digital signals by A/D converters 113 and 114, whereby signals 145 and 146 are obtained (see, for example, Japanese Unexamined Patent Application Publication No. 09-168037 (FIG. 3)).
In the above-described conventional configuration, even if there is no level difference between the reception signals input to one mixer 103 and the other mixer 104, when there is a difference, for example, in conversion gain between mixer 103 and mixer 104, and similarly, when there is a difference in gain between amplifier 109 and amplifier 110, a level difference occurs between the baseband I signal input to the A/D converter 113 and the baseband Q signal input to the A/D converter 114. In this state, conversion of each baseband signal into a digital signal by each A/D converter causes bit errors.